Subdivision tears and cleaning up edges
Check your topology. You have N-gons and triangles. Stick to rectangles so the subdivision can work properly. Use MatCap's for a better view of lightning around the edges when adding new loop cuts so you dont have to go to modeling hell.

I have fixed your cube for you. I'm still using Blender 2.79 so you can import the fbx from here: https://drive.google.com/file/d/1_hVeMLHYu2jdWg9_t7FFv98-NyJLxZTO/view?usp=sharing